The invention relates to a compacting granulator for continuous, compacting granulation and possible simultaneous mixing of at least one powdery or pasty material under supply of at least one fluid in a drum.
In many industries producing or handling powdery materials a granulating or agglomerating treatment is performed for making the materials less dusty, to increase their flowability, to make them easier to dose or in general easier to handle or to give them a desired appearance, by which treatment the primary particles and/or small agglomerate granules are united for the formation of bigger agglomerate granules. It may also be of interest to produce granulate on the basis of pasty starting materials, possibly with the addition of dry, powdery components.
Examples of such industrial branches, in which such processes take place, are the detergent industry, the fertilizer industry and the food, drink and tobacco industry as well as the pharmaceuticals industry and the metallurgical industry.
The attainment of agglomerated particles is achieved by use of widely differing measures either as part of a drying or production process or as a pre-treatment and by use of binding forces which are either present spontaneously or provided by the supply of suitable liquid or vaporous auxiliary means.
Many agglomeration processes, as for instance those performed in connection with a spray drying or a subsequent treatment in a fluidized bed, however, entail a reduction of the powder density of the material, also called the bulk weight, which in respect of a lot of products is undesirable, i.e. on account of increased packing and transport costs. To this may be added that the consumers wishes for instance within the detergent area tend to go towards more concentrated products, for the production of which components with relatively high powder density are used.
It is known that in connection with the selection of detergent components in a particular ratio, by mechanical treatment of the powdery material derived from spray drying by addition of a tenside acting as a binding agent, a detergent agglomerate with increased powder density is obtained, presumably because the mechanical treatment results in dense agglomerates with comparatively little interspace air between the primary particles.
Such a treatment is disclosed in published Danish Patent Application No. 1823/89, in which it is proposed to perform the agglomeration or the granulation while the material passes through a horizontal or slightly tilting mixing drum, in the interior of which a central, rotatable shaft is provided, the shaft carrying several radially directed impact tools. The mixing drum itself is stationary, and the passage of the material through the drum is therefore exclusively controlled by the impact tools.
Related apparatuses are known which have a horizontal, rotating mixing drum, in which a stirring shaft, which is parallel to the axis of the cylinder, but positioned under and at the side thereof, rotates in the same direction as the drum, see Japanese Patent Application 79-23438 (publication no. 80-116690). The described apparatus is for use in the manufacture of phosphate fertilizer with a surface layer of water-soluble phosphoric acid, the most important task being to distribute the liquid phosphoric acid on the phosphate granules. Inter alia as a consequence of the fact that the mixing shaft is adapted to rotate fairly slowly and that the design of the apparatus does not make an effective control possible of the staying time of the material in the apparatus and consequently the particle size, it will be obvious that this known apparatus is not suited for a real compacting granulation for the attainment of dense agglomerate particles with high density. Furthermore, the apparatus is not suited for treating material which consists of so small particles, that it is not flowing freely.
In addition to this many mixing apparatuses are known, in which large shear forces are exerted on the material to be mixed (high shear mixers) and which are provided with a fast rotating impeller, which apparatuses may also be used for agglomeration, for instance in connection with the spraying of a suitable liquid. However, it is here substantially the question of apparatuses working batchwise and which are less suited for industrial production on a large scale.
Within the detergent industry the apparatuses used for agglomeration and compacting comprise i.a. an apparatus which combines a mixing in a V-shaped chamber with the impact from an impeller. The compacting ability of these apparatuses does not, however, reach the desired level.
A granulator is required which combines the following properties or characteristics:
1. Continuous operation PA1 2. The possibility of obtaining more compact agglomerates than can be obtained with known apparatuses with a single passage of the material to be treated PA1 3. Big production capacity PA1 4. The possibility of obtaining a narrow granule size distribution, and PA1 5. General applicability for treatment of very differing materials without operational complications. PA1 i) a drum rotatable around a substantially horizontal longitudinal axis, said drum having a length which is bigger than its diameter, and means for introducing pasty or powdery material at one end and means for removing granulate at the opposite end, PA1 (ii) a shaft extending parallel with the longitudinal axis of the drum through the length of the drum placed eccentrically in the drum in a level below the longitudinal axis of the drum and displaced laterally in the rotational direction of the bottom part of the drum relative to the vertical plane through the longitudinal axis, said shaft being at at least the major part of its length present in the drum provided with compacting means, the parts of which that are farthest away from the shaft passing, when the shaft is rotating, through an area near the part of the drum wall which is closest to the shaft, PA1 iv) means for rotating the drum around its longitudinal axis as well as means for rotating said shaft in the same direction and with a number of revolutions, which may be adjusted to a value which during the running of the granulator is from 10 to 1000 times bigger than the number of revolutions of the drum for hurling up the tumbled material present in the drum such that it is thrown through a path outside the area, where the product would be present in the drum, was it not influenced by the compacting means, PA1 iv) at least one scraper arranged stationarily in the drum for removing material from the inner wall of the drum for uniting it with material uphurled by the compacting means and PA1 v) at least one nozzle for distributing at least one fluid in the part of the drum which is passed by the material in uphurled state.
It has turned out that the possibility of meeting the requirements 2 and 4, i.e. the attainment of particularly compact agglomerates with narrow granule size distribution, does not solely have as a prerequisite that the mechanical conditions are present, in this respect the sufficient working energy, to obtain an effective compacting, but also to a very high extent the prerequisite of imparting to the material being treated a particularly homogenous moistening or contact with the agglomeration fluid used in the process.
In the known apparatuses of the type comprising a drum, the granulation fluid is introduced in form of a liquid or vapour, the latter being condensed on the powdery material, by means of nozzles which are either placed where the very powder mass is present during the operation of the apparatus or placed above the powder and adapted to distribute fluid on the surface formed by the powdery layer during its tumbling in the drum. Since the speed of the turning over of the powder is limited, the distribution of the liquid sprayed on or condensed becomes insufficient in the powder mass, the particles close to the surface of the powder layer being strongly moistened, whereas the particles positioned deeper in the powdery layer will have to participate in the tumbling process caused by the rotation of the drum for a long time before ending in the surface layer, where the moistening takes place. Thus, there are in the known apparatuses far from optimum conditions in respect of obtaining a homogenous moistening.
A more homogenous contact between the material in the drum and the agglomeration fluid blown in is achieved when this contact takes place while the material is projected in the air space in the drum, such that the powdery particles do not touch each other at the time of contact. As in the case in the known agglomeration processes in a fluidized bed, a considerably more homogenous moistening is obtained than would be the case if it took place while the particles were in mutual contact. When a particle projected in the air contacts a drop of liquid, the drop is evenly distributed on the surface of the particle. If, however, a drop of liquid hits a material which is in such a condition that the particles are in contact, the liquid will first and foremost penetrate into the interspaces between the particles and cause a very random and inhomogeneous conglomeration thereof.
This perception is used in the granulator according to the invention which better than any known apparatus is capable of meeting the combination of requirements listed above under 1-5. This granulator is according to the invention characteristic in comprising